1. Field of the Invention
The present invention relates to an image reducing apparatus for obtaining a reduced image from an image of an original size.
2. Related Background Art
Hitherto, a sub-sampling method, a projecting method, a decision-of-majority method, an error diffusion method, or the like has been used as means for obtaining a reduced image from an image of the original size. The projecting method is constructed as shown in FIG. 1.
Reference numeral 200 denotes an image of an original size such as a binary image; 201 indicates a line buffer which stores pixels (reference pixels) to be filtered and constructs a pixel block; and 202 a low pass filter for determining an objective pixel in accordance with the sampling period corresponding to a reduction ratio and for executing a smoothing process by using the objective pixel as a center. A projection value calculator to output a projection value is constructed by the line buffer 201 and low pass filter (LPF) 202. Reference numeral 204 indicates a comparator for comparing the projection value with a threshold value T and binarizing. The threshold value T is stored in a threshold value memory 206. Reference numeral 205 indicates a reduced image obtained.
The operation in the case of reducing the image size to 1/2 in each of the vertical and lateral directions will now be described as an example. In FIG. 2A, reference pixels which are used in the projecting method are shown by being surrounded by a bold frame line, in which a pixel C is set to a center pixel. Each reference center pixel is shown by a black dot. Assuming that the pixel values (0 or 1) of the reference pixels are set to D.sub.1.1 to D.sub.3.3 as shown in FIG. 2B, a projection value sum by the LPF 202 is obtained as follows: ##EQU1## When sum .gtoreq.T, the comparator 204 generally outputs "1" as a binary threshold value by using T=8. When sum &lt;T, "0" is output.
The reduced image of 1/2 is obtained by the above operations.
On the other hand, there has been also known an image reducing method whereby a smoothed image is obtained from an image of an original size and, by sampling the pixels of the smoothed image, a reduced image is obtained. The projecting method is constructed as shown in FIG. 3.
Reference numeral 301 denotes an image of an original size and is a multivalue image; 302 indicates a low pass filter (LPF); 303 a sub-sampling unit; and 304 a reduced image derived by the device. The input image of the original size is subjected to a low pass filtering process which differs depending on the reduction size by the LPF 302, thereby obtaining the smoothed pixels. The number of pixels necessary to construct the reduced image 304 among the smoothed pixels is obtained by the sub-sampling unit 303. On the other hand, in the case where the pixels of the input image have binary values, as shown in FIG. 4, a binarization unit 305 is added after the sub-sampling unit 303 in FIG. 3 and the multivalue data which was processed by the LPF is again binarized, thereby obtaining the reduced image of the binary values.
The case of reducing the image size to 1/2 in the vertical and lateral directions will now be described as an example for convenience of explanation. In FIG. 5, reference pixels to be filtered by the LPF in the image of the original size are shown by being surrounded by a bold frame line, in which a pixel 310f is set to a center pixel. The values of pixels 310f, 310a to 310c, 310e, 310g, and 310i to 310k are x.sub.0 to x.sub.8 as shown in FIG. 5, respectively. An output X.sub.0 of the LPF 302 is expressed as shown in the equation (2) by using x.sub.0 to x.sub.8 : ##EQU2##
Among the results of the outputs of the LPF, the results at the positions indicated by a mark .smallcircle. in FIG. 5 are sub-sampled by the sub-sampling unit 303, so that the reduced image of 1/2 can be obtained.
On the other hand, in recent years, since the sequential reproduction encoding system is effective for retrieval of the data base or the like, a hierarchy encoding as shown in FIG. 6 is executed. The image in a frame memory 101 to store the image of the original size is reduced by using a reducing apparatus 102 and the reduced image is stored into a frame memory 103. A reducing apparatus 104 receives the reduced image from the frame memory 103 and further reduces it and the reduced image is then stored into a frame memory 105. By using those reduced images, an encoder 108 encodes the smallest image stored in the frame memory 105 and transmits the encoded image. Next, efficient encoding is executed by an encoder 107 by using both the smallest image stored in the frame memory 105 and the image stored in the frame memory 103, and the result is transmitted. Finally, the images in the frame memories 101 and 103 are encoded by using an encoder 106 and the result is transmitted. In this manner, the information of all resolutions is transmitted. Generally, the same type is used as the reducing apparatuses 102 and 104.
However, according to the above reducing system, in the case where the image is a character image including many thin lines or the like, in the reduced image, the thin lines are cut out or not regenerated, so that the image quality is remarkably deteriorated. In addition, the reproducibility of a pseudo-gradation image by a dither method or an error diffusion method is not so good, as well.
That is, the construction of FIG. 4 to reduce a binary image will now be described as an example for simplicity of explanation. For an oblique thin line as shown in FIG. 7A, if the low pass filtering using the reference pixels in FIG. 5, the sub-sampling of the pixels shown by the mark .smallcircle., and the binarization using the threshold value of 0.5 are executed, such an oblique thin line is not regenerated because the output value of the LPF is "0.37" at most. On the other hand, a thin line as shown in FIG. 7B is regenerated if the positions of the pixels to be sub-sampled are located on the thin line. However, if those positions are elsewhere, such a thin line is not regenerated, in a manner similar to the case of FIG. 7A.